A coaxial loudspeaker has two sound producing elements—a low frequency (LF) element and a high frequency (HF) element, each of them having their own moving assembly including a voice coil. The LF and HF elements have their central radiation axis coincident, so that they produce a coherent wave front.
Broadly speaking there are two types of coaxial loudspeaker. In one, the HF element radiates according to the principle of what is known in the industry as compression drive which relies on a horn to guide the HF wave propagation, resulting in controlled directivity and enhanced sensitivity by acoustic impedance matching. In some implementations the role of the horn is played by the membrane (cone) of the LF element, in others a dedicated horn, inserted in front of the LF cone, is used. In the second type of coaxial loudspeaker the HF element simply comprises a conventional tweeter mechanically secured in front of the LF cone; and the HF element therefore works as a direct radiator as opposed to being horn loaded like in the first type discussed in the preceding paragraph.
If the coaxial concept has some inherent technical advantages, the constraints it brings tend to result in higher manufacturing costs. One of them is the requirement for each coil (LF or HF) to operate in a separate magnetic annular gap, the two gaps having to be concentric to fulfill the condition for coincident radiation axes.
Several methods have been tried in the past to simplify the magnetic assembly of coaxial drivers in order to make them more cost-effective, the first obvious goal being to use only one magnet instead of two.
An early example of a coaxial design using a single magnet is described in U.S. Pat. No. 2,539,672 (1951) by H. Olson. However it should be noted that the permanent magnet used by Olson has a cylindrical shape, and occupies a central location in the mechanical structure. Although it is not explicitly stated by Olson in his patent, it is clear that the magnet material is Alnico, which was the most popular material for loudspeaker magnets at the time. The cost of this material has now become prohibitive such that no cost-effective design can be based on this material nowadays. Today, the two magnet materials commonly in use are ferrite (inexpensive but bulky) and neodymium (very compact and expensive). Ferrite, the less expensive solution, practically dictates the use of a ring shaped magnet.
A later patent by A. Garner (U.S. Pat. No. 4,256,930, 1981) describes another coaxial driver design using a single permanent magnet. In Garner's design, shown in FIG. 4, the magnet 101 is ring shaped as has become the norm for ferrite magnets. As far as the HF element is concerned, this design is of the compression drive type, with a moving diaphragm 110 radiating inwards into air channels through a pole piece 108. These channels extend to a central passage 109 forming a horn as it flares outwards. With respect to the direction of propagation, the HF diaphragm 110 forms a concave dome, which is a very common feature that suits this particular driver design. This is directly related to the fact that the diaphragm 110 is located behind the magnetic assembly made of ring magnet 101, top plate 102, pole piece 108, and rear plate 103. Similarly the air gap intended to receive the voice coil 112 is located at the rear of the magnetic assembly.
Although Garner's design is still popular, it has an inherent shortcoming in the use of the concave dome design 110. In particular, the HF sound eventually propagates within the waveguide as a portion of a diverging spherical wave, but with a concave dome it has to start as a converging wave, then reach an acoustic focal point before expanding again. This adds to the propagation path, and is detrimental to a proper time alignment between the LF and HF elements. In addition, distortion is increased as in this initial propagation path the wave travels with a high acoustic intensity due to the narrow cross-sectional area.
Other more recent attempts to design a single magnet coaxial loudspeaker include the one proposed by J. Peng in U.S. patent 2003/0206641 (2003), which is illustrated in FIG. 5. As shown, the magnetic assembly comprises a ring-shaped magnet 155, a reversed T-shaped pole piece 153 forming together with outer C-shape plate 154 a dual air gap for voice coils 151 and 152. It appears from the drawings of Peng that the HF element of this driver is of the direct radiating type, rather than the compression drive type. The simplicity of this design however relies on the magnet being an internal part of the HF structure, but this limits the size of the magnet and as a result is likely to dictate the use of neodymium which is an expensive material.
The present invention aims to provide an alternative magnetic assembly for a loudspeaker design and to provide an alternative coaxial loudspeaker design using the new magnetic assembly.